This section provides background information related to the present disclosure which is not necessarily prior art.
A wide variety of apparatuses have been proposed for the fluxing and mixing of thermoplastic materials. The essential requirements for such apparatus include rapid fluxing or melting of the material and efficient mixing of the material components into a homogeneous blend, both at an effectively high throughput rate. Whereas some prior apparatuses are capable of satisfying the desired fluxing and mixing requirements, they are incapable of delivering the necessary throughput rate. Other prior apparatuses sacrifice fluxing and/or mixing efficiency in order to provide the required throughput rate.
There are several different plastic melt mixing devices that can be attached to the feed screw for the mixing of thermoplastic materials with high fluxing and mixing efficiency and are capable of delivering a high material throughput rate. Dispersive mixing applies force to the materials and thus requires drive energy that ends up in the polymer mix to help melt it and/or raise its temperature. Consequently, dispersive mixing assists or adds to the melting capacity of the screw. In addition, tight clearances often function as a “dam,” restricting unmelted polymer from passing through until reduced in size or melted. Many feed screw designs would discharge unmelted polymer at almost all speeds without a dispersive mixer. The “Maddock” mixer is an example of a mixer that is primarily dispersive, with lesser distributive characteristics.
The U.S. Pat. No. 3,730,492 describes the “Maddock” mixing head. An extruder heats thermoplastic material to a flowable condition, longitudinally advancing the heated material under pressure toward the discharge end with a rotating extrusion feed screw having the Maddock mixer head at the downstream end thereof. The mixer head divides the heated material into a plurality of streams and passes the streams through a plurality of longitudinal mixing passages thereby passing the plurality of individual streams of the partially fluxed melt through high shear zones between the mixer head and the barrel of the extruder, and then passes the fluxed melt into the interior of the mixer head and therethrough to the discharge end of the mixer head. In summary, the Maddock mixer passes melt over a very narrow clearance where it experiences high shear for a high degree of dispersive mixing. The melt is divided several times and reoriented to provide some distributive mixing.
Although distributive mixing also requires some drive power, it is generally small enough to have a very minor effect on the melt temperature. A “Saxton” style mixer is an example of a mixer that is mostly distributive with minor dispersive characteristics. In the Saxton mixer, melt is divided many times and recombined with numerous reorientations to provide mostly distributive mixing.
An “Eagan” style mixer combines strong dispersive and distributive characteristics. All of the flights are undercut so there is a lot more dispersive and distributive mixing than with the Maddock mixing section. In the Egan mixer, a reduced diameter provides multiple high-shear regions as well as leakage flow and many reorientations for high levels of both dispersive and distributive mixing.
U.S. Pat. No. 4,779,989, issued to Robert A. Barr, describes a mixer assembly for mixing fluid material in the bore of a barrel including a structure for mixing and pumping toward its outlet end the material fed thereto. A stator assembly is fixed in the barrel having outer cylindrical surface portions conforming substantially to the surface of the bore and having a stator bore provided with a groove-interrupted inner surface, and a driven rotor member has a groove-interrupted outer surface confronting the inner stator surface. The inner stator surface and outer rotor surface each have continuous helical grooves cut therein along a helical path which changes in depth with length from a small minimum depth to a larger maximum depth but never disappearing and having a plurality of axially spaced circumferential grooves at planes perpendicular to the bore center axis subdividing such surfaces into axially spaced serially arranged mixer sections with portions of the helical grooves extending between the circumferential grooves defining each section.
U.S. Pat. No. 5,988,866, issued to Robert A. Barr, describes a mixer for plasticable resins having a fixedly positioned heated barrel with a power driven feed screw mounted axially in the barrel bore, and a driven rotor axially aligned with and extending in a downstream direction from the downstream end of power driven screw. A series of elongated rotor flow transfer cavities are in the outer surface of the rotor. The rotor flow transfer cavities extending inwardly and are arranged in a plurality of axially aligned rows and a plurality of annular rows concentric to the axis of the rotor. A floating sleeve is coaxially positioned over the power driven rotor and interposed between the rotor and the barrel so as to be capable of independent rotation relative to both the power driven rotor and the barrel. The floating sleeve has a plurality of parallel outwardly extending ring flanges extending radially outwardly and inwardly facing a plurality of annular and elongated in cross-section outer sleeve flow channels each having an upstream end and a downstream end provided between adjacent ring flanges. A series of outflow apertures extend through the floating sleeve and communicate on opposite ends with the upstream end of an elongated outer sleeve flow channel of the floating sleeve flow channel and the downstream end of one of the elongated rotor flow transfer cavities of the rotor.
U.S. Pat. No. 6,254,266, issued to Robert A. Barr and Jeffrey A. Myers, describes an extruder-mixer having a plurality of rotor rings provided on the downstream end of a motor driven feed shaft mounted for rotation in a conventional heated barrel or stator. The rings comprise a plurality of spaced driven rotor rings spaced apart from each other and a plurality of non-driven but rotatable floating rings interleaved between each pair of driven rotor rings. Both rings have parallel upstream and downstream faces between which polymer flow passageways extend so that the polymer melt moves downstream first through one type of ring followed by movement through the other type of ring and the viscosity of the melt causes the rotatable floating rings to be rotated by the driven rings at a slower speed than the driven rings so that shearing force on the melt effects mixing of the melt.
The above-described plastic melt mixing devices all are attached to the feed screw for rotation. Since the rotational speed of the mixer attached to the screw is the same as the screw speed the mixing effect cannot be changed without a redesigned mixing head. This can be a problem, especially with large extruders wherein the screw rotational velocity (i.e. RPM) is relatively low compared to smaller diameter screws. This leads to less of a mixing effect. To provide for an adjustable mixing effect with a given design mixer it would be desirable to have the mixer rotational velocity (RPM) adjustable independent of the extruder screw.
In addition to mixing devices, it would be beneficial to drive other devices independently of the feed screw. Such devices can include, for example, vent sections, injection port sections, melting sections and temperature gradient reduction sections.